in lib/common.sh add some brackets near line 385:
NEWSIZE=$(($BLOCKSIZE*$NEWSIZE/1024))
and in the case you installed a native language like me (i installed german), you have to fix the grep a line above:
BLOCKSIZE=$(resize2fs $LOOP $NEWSIZE"M" | grep "Das Dateisystem auf" | awk '{ print $(NF-2)}')